Filters and respirators worn over the face are used as protection against toxic vapors in many occupations. At present there is no fast and accurate way to determine the status of a used or partially used respirator device. Proposed approaches to estimating the useful life have included accurate logging of use time, periodic breakthrough testing, and color change indicators. These methods merely estimate the status of the respirator device and, as is the case with the breakthrough testing, often result in the exhaustion of the filter thereby making the respirator useless. The ideal solution to this problem consists of a filter cannister which incorporates an indicator or alarm which signals the end of the respirator's useful lifetime.
Ideally, a sensor for detecting the exhaustion of an adsorbent bed in a respirator should detect all vapors the adsorbent is designed to remove. Further, the sensor is required to operate over the entire range of temperatures and pressures which are normally encountered in adsorbent use. This means that the sensor must be capable of operating in temperatures from -65.degree. to 110.degree. F. and pressures from 0.8 to 1.2 atmospheres for personal protection applications and can be designed for severe pressure, vacuum, humidity and temperature conditions present in industrial and military use. The sensor must also be capable of enduring all conditions of use for an adsorbent such as being attitude insensitive, shock and vibration resistant, storage stable for as long as the adsorbent is stored and the sensor must also last as long in use as the adsorbent. Finally, the sensor must have a response time which gives the user sufficient warning of adsorbent bed exhaustion.
One approach to this problem is to detect the presence of all possible toxic gases that could emanate from the adsorbent by using a sensing device which is sensitive to all the toxic gases which the adsorbent bed is designed to adsorb. An example of this method can be seen in U.S. Pat. No. 3,902,485 (Wallace) issued on Sept. 2, 1975. In this method, spaced electrodes, at least one of which is coated with a basic nitrogen-containing polymer of high electrical resistance, project into an electrical conducting medium such as activated charcoal in a container. The coated electrode is connected in series with signalling means which puts out an audio and/or visual signal. The coating on the electrode forms an electrically conducting quaternary ammonium salt in the presence of selected and predetermined toxic gases to thereby lower the electrical resistance of the polymer coating and complete the electrical circuit between electrodes through the charcoal. This activates the signalling means to generate an alarm signal.
Sensors designed for detecting the presence of toxic gases for use in combination with conventional gas filter breathing apparatus suffer from several drawbacks. First, these sensors are generally relatively expensive when compared with the cost of the adsorbent bed. Second, the typical sensors are only sensitive to a few or several of the potentially toxic gases, i.e., the sensors are somewhat selective in their response to gases and vapors. Further, sensors detecting gases in the adsorbent bed output must detect everything and anything that comes through the bed. This is a difficult problem since it is nearly impossible to predict what toxic gases the respirator adsorbent and the user of a respirator may be exposed to. This makes design of sensors capable of detecting everything that comes through the bed very difficult and virtually impossible. In addition, sensors detecting the presence of toxic gases in the adsorbent bed will have significantly different reactivity than the adsorbent bed itself for the same gas or vapor. This will often cause premature signalling of adsorbent bed exhaustion or, even more dangerous, the alarm signal will be generated too late and toxic gases will pass through the adsorbent bed to the user.
Accordingly, there is a need in the art for an improved sensor device which can be used in combination with an adsorbent bed material to provide a real-time warning of the exhaustion of the adsorbent bed and thereby prevent human exposure to harmful vapors. In addition to having the appropriate analytical response described above, the sensor must also be low-cost, low-power, tiny, stable, rugged and completely reliable.